1. Field of the Invention
The invention relates to an internal combustion engine including a secondary air supply mechanism, a control apparatus for the internal combustion engine, and a control method for the internal combustion engine.
2. Description of the Related Art
A conventional exhaust gas control structure is known, in which an exhaust gas control catalyst is provided in an exhaust pipe of an internal combustion engine. The exhaust gas control catalyst has a function of transforming air pollutants such as HC, CO, NOx, and the like that are contained in exhaust gas into harmless H2O, CO2, N2, and the like.
However, the exhaust gas control catalyst needs to be placed in an environment at a given activation temperature (for example, 350° C. or higher) so as to fully perform the function. Therefore, when the engine is cold, for example when the engine is started, the exhaust gas control catalyst does not fully perform the function.
Thus, there is known an internal combustion engine including a secondary air supply mechanism for fully purifying the exhaust gas (for example, refer to Patent Document 1).
[Patent Document 1]
Japanese Patent Laid-Open Publication No. 11-229861
The secondary air supply mechanism is a system which supplies secondary air to an upstream side of an exhaust gas control catalyst in an exhaust pipe via a secondary air supply passage by using an air pump.
The mechanism supplies secondary air into the exhaust pipe so as to increase a concentration of oxygen in the exhaust pipe, thereby oxidizing HC and CO contained in the exhaust gas so as to purify the exhaust gas. Also, by oxidizing HC and CO, the temperature of the exhaust gas can be increased. Therefore, there is also an effect of reducing the time required for an environmental temperature for the aforementioned exhaust gas control catalyst to reach the activation temperature.
When the secondary air is supplied, since the air pump is operated, electric load increases. Therefore, load for electric power generation performed by an alternator increases, and a load on the engine increases. As a result, the engine speed decreases.
When the engine speed thus decreases, idling stability deteriorates during idling immediately after the engine is started when the engine is cold. Also, there is a possibility that the engine stall occurs due to the deterioration of the idling stability.
Also, the exhaust gas amount decreases due to the decrease in the engine speed. Accordingly, there are problems that the effect of the secondary air supply decreases, warm-up efficiency of the catalyst decreases, and the exhaust gas characteristics deteriorate.